This invention pertains to a burglar alarm and more particularly to such alarms which are self-contained and can be selectively defeated. Such alarms can be classified according to the method employed to detect the burglar or intruder. In the first class are passive devices which are energized by the closing of one or more control circuits. However, the devices of this class can be defeated by cutting or opening the control circuits. Another class is generally the wireless type which uses acoustic standing waves, electromagnetic fields or photoelectric techniques. Devices of this class are usually complex, expensive, cumbersome and require considerable electric power. A third class relies on the opening of a normally closed control circuit to activate the alarm. Conventional devices of the third class commonly use a relay as a means for detecting an opening of the control circuit. When the control circuit is open the relay "drops out" to energize an alarm circuit. However, relays and even very sensitive relays require considerable continuous current. Hence, they are powered by house current or by batteries which must be frequently changed or recharged.
In addition with many alarm systems it is often necessary for the owner to enter the area guarded by the alarm device without triggering it. Therefore, such alarms should include confidential or coded defeating mechanisms.